244 ELEMENTARY PHYSIOLOGY less. 



neutrul, of the solution in which they are at work.^ (iv) 

 Their action stops in presence of an excess of the special 

 products of their activity, and (v) It has not so far been 

 conclusively proved that the enzymes are themselves used 

 up during the changes which they produce on other 

 substances. 



Nearly all the chemical changes which the food under- 

 goes in the alimentary canal are brought about by the 

 action of these soluble ferments or enzymes. 



9. The Structure of the Stomach. — The stomach, 

 like the gullet, consists of a tube with muscular walls 

 composed of smooth niuscular fibres, and lined by nmcous 

 membrane ; but it differs from the gullet in several cir- 

 cumstances. In the first place, its cavity is greatly larger, 

 and its left end is produced into an enlargement which, 

 because it is on the heart side of the body, is called the 

 cardiac dilatation (Fig. 76, h). The opening of the 

 gullet into the stomach, termed the cardiac aperture, 

 is consequently nearly in the middle of the whole length 

 of the organ, which presents a long, convex, greater 

 curvature, along its front or under edge, and a short, 

 concave, lesser curvature, on its back or upper con- 

 tour. Towards its right extremity the stomach narrows, 

 and, where it passes into the intestine, the muscular 

 fibres are so disposed as to form a sort of sphincter around 

 the aperture of connuunication. This is called the 

 pylorus (Fig. 76, d). 



The nuiscular coat of the stomach, consisting of un- 

 striated muscular tissue, is made up of two layers, an 

 outer longitudinal and an inner circular, together with 

 a certain amount of muscle fibres which are continuous 

 with the circular fibres of the cjesophagus and which, 

 running obliquely, merge into the internal circular 

 layer of the stomach. The mucous membrane which 



1 Thus the pepsin of gastric juice acts best in presence of hydro- 

 chloric acid and the trypsin of pancreatic juice in presence of sodium 

 carbonate. 



